We propose to better understand the mechanisms of action of the v-erb A oncogene, and to relate the role of v-erb A in viral neoplasia to events in normal differentiation and in human disease. V-erb A, an oncogene in the avian erythroblastosis retrovirus genome, participates in erythroid leukemogenesis and alters the growth properties of infected fibroblasts. V-erb A is a mutant, transduced copy of a normal cell gene (c-erb A) encoding a thyroid hormone receptor. Thyroid hormone receptors are ligand-regulated transcription factors, and belong to a larger family of nuclear receptors that play key roles in vertebrate development, homeostasis, and differentiation. Naturally occurring mutations in nuclear receptors have been implicated in important human endocrine and neoplastic diseases, including hepatocellular carcinoma, acute promyelocytic leukemia, renal clear cell carcinoma, and resistance to thyroid hormone syndrome. In many cases these mutant receptors, like v-Erb A, can act as dominant negatives. An important component of our research extends to understanding the molecular defects in these human diseases, and why different receptor mutations result in different diseases. We propose four specific aims to investigate these questions: SPECIFIC AIM A. We will identify v-Erb A and PML-RARalpha target genes. SPECIFIC AIM B. We will elucidate how v-Erb A represses target gene expression once bound to a target DMA SPECIFIC AIM C. We will elucidate how v-Erb A function is altered by other signal transduction pathways operative in the erythroleukemic cell SPECIFIC AIM D. We will use v-Erb A as a model to improve our understanding of the actions of the aberrant nuclear receptors involved in human disease.